Feature summary (what you would like to be able to do and where):
As proposed in https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(proposals)&diff=prev&oldid=1243002498, it would be useful for an event coordinator to declare the IP address(es) which will be used during an event, and have this information be visible in the checkuser tool (and through the checkuser API).

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

This came up recently while I was investigating what appeared to be a bunch of vandalism-only socks. There were a bunch of new accounts, all making non-useful edits, from the same IP address. I ended up blocking them. Unfortunately, it turned out that this was a bunch of 13 year old kids who were attending a completely legitimate "Learn to use Wikipedia" event at a public library. Had there been some notification that came up in the checkuser tool, alerting me to the fact that an event was in progress using that IP, I would have not performed the blocks.

This is not the first time this sort of thing has happened, and it won't be the last. I had a similar experience last year when I found a LTA (long-term abuser) active on a given IP. The only thing that clued me in was that I recognized many of the other accounts using that IP so I investigated further. It turned out to be the venue where Wikiconference North America 2023 was being held!